1. Field of the Invention
This invention relates to lightning current responsive alarm devices, and more particularly to a device of the kind above described which is suitable for alarming the operation of a protective coordination gap across which lightning current of large value flows to be shorted to earth.
2. Description of the Prior Art
Lightning current responsive alarm devices employed hitherto with lightning arresters have generally had a structure as shown in FIG. 2. Referring to FIG. 2 showing the prior art structure designated generally by the numeral 10, a plurality of series-connected non-linear resistors R1 and R2 are connected in parallel with an air gap G, and the parallel connection of the non-linear resistors R1, R2 and air gap G is connected between earth and a protective coordination gap or lightning arrester A disposed in a power transmission system. A capacitor C and a counter coil CC of a sensor or counter 20 are connected in parallel with one of the plural non-linear resistors, for example, the resistor R2. Such a lightning current responsive alarm device is disclosed in, for example, Japanese Utility Model Publication No. 8042/1959 which has been granted as Utility Model Registration No. 501176.
In the prior art structure shown in FIG. 2, the non-linear resistors R1 and R2 are of the kind having relatively little non-linearity, and the air gap G is arranged to conduct the follow current. Therefore, the correlation between the drop voltage appearing across the non-linear resistors R1, R2 and the gap length of the air gap G has been such as to limit the alarmable range of lightning current to a narrow one as described later, and the alarm device has been unable to satisfactorily provide an alarm up to a large current region of scores of thousands of amperes. Further, the prior art lightning current responsive alarm device has been defective in that it is not capable of reliably responding to a follow current such as an oscillatory follow current produced as a result of induction between conductors of electric power lines. The prior art alarm device has also been defective in that the useful life of the air gap G tends to be excessively shortened by being impaired by the short-circuit follow current which has a large current value of order of thousands of amperes among various kinds of follow currents.
FIG. 3 shows the operating performance of the prior art lightning current responsive alarm device shown in FIG. 2. Referring to FIG. 3, the curve G1 represents the operating performance observed when the gap length of the air gap G is set at such a value that discharge can take place at the minimum operating current level of the counter 20. In such a case, a relatively slight increase in the current value results in a great increase in the drop voltage across the non-linear resistors R1 and R2, and the resultant drop voltage is as high as several times that corresponding to the minimum operating current level of the counter 20. As a results, the input voltage Va applied to the capacitor C connected to the connection point of the non-linear resistors R1 and R2 in parallel with the counter coil CC is cut off in a short period of time due to the presence of the air gap G and fails to attain the level which is sufficient to provide the maximum input voltage Vo for the counter 20. Thus, the input voltage Va drops abruptly as seen in FIG. 3, with the result that the operating current region of the counter 20 is limited to a very narrow one as shown by A1. The input voltage Va applied to the capacitor C drops similarly abruptly, and the operating current region of the counter 20 is similarly limited to a very narrow one as shown by A2 in FIG. 3 when the gap length of the air gap G is set at another value as represented by the curve G2. It will thus be seen that the operating current region of the counter 20 is limited to a very narrow range which is dependent on the gap length of the air gap G. Impossibility of extensive widening of the operating current region of the counter 20 has been experimentally confirmed. This is principally because an excessive increase in the gap length of the air gap G is undesirable in that the counter 20 cannot measure the lowest possible minimum operating current, and the maximum input voltage Vo of the counter 20 will be exceeded.